A tattoo is a marking made by inserting indelible ink into the layers of skin to change the pigment for decorative or other reasons. By virtue that a tattoo printed into the skin, it will become a permanent mark. The skin is definitely altered, whether it is harmed is another matter. Health risk is another matter.
Because it requires breaking the skin barrier, tattooing may carry health risks, including infection and allergic reactions. Modern tattooists reduce such risks by following universal precautions, working with single-use items, and sterilizing their equipment after each use. Many jurisdictions require that tattooists have blood borne pathogen training, such as is provided through the Red Cross and OSHA.
Infection from tattooing in clean and modern tattoo studios employing single-use needles is rare. In amateur tattoos, such as those applied in prisons, however, there is an elevated risk of infection. Infections that can theoretically be transmitted by the use of unsterilized tattoo equipment or contaminated ink include surface infections of the skin, herpes simplex virus, tetanus, staph, fungal infections, some forms of hepatitis, tuberculosis, and HIV. No person in the United States is reported to have contracted HIV via a commercially-applied tattooing process.
Tattoo inks have been described as "remarkably nonreactive histologically". However, cases of allergic reactions to tattoo inks, particularly certain colors, have been medically documented. Occasionally, when a blood vessel is punctured during the tattooing procedure a bruise/hematoma may appear.